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As we know in football a goal is valid, if it goes through the goal-line. However, referees need(ed) to decide a goal's validation with their eyes. We can imagine how hard this is even if referees have assistant referees. So eventually we created technological solutions to this problem. The first one is the Hawk-Eye System. It's Sony's property, which uses a few fast cameras. The other tech is GoalControl, a German system.